Since plastics optical materials are lightweight, hardly breakable, and excellent in dyeability as compared with optical materials made of inorganic materials such as glass, plastic materials of various resins are widely used as optical materials for eyeglass lenses, camera lenses, and the like. Recently, there has been a demand for high performance of optical materials having such properties as a high transparency, a high refractive index, a low specific gravity, a high heat resistance, and a high impact resistance.
Polythiourethanes are widely used as an optical material by virtue of their excellent optical characteristics and excellent mechanical properties. Polythiourethanes may be prepared by reacting a thiol and an isocyanate. Lenses produced from polythiourethanes are widely used since they have a high refractive index, a lightweight, and a relatively high impact resistance.
Isocyanates used as a raw material of a polythiourethane are capable of producing polythiourethanes having different structures depending on the number and position of the functional groups in the isocyanates. Thus, the isocyanates have a significant impact on the physical properties of a product produced from the polythiourethane. Accordingly, a certain kind of isocyanate that can impart the desired properties to a final product is used.
Especially, since xylylene diisocyanate (XDI) has both characteristics of alicyclic isocyanates (e.g., resistance to yellowing, readily controllable reactivity, and the like) and those of aliphatic isocyanates (e.g., excellent mechanical properties, high refractive indices, and the like), it is possible to materialize excellent properties of an optical lens by taking advantage of the respective characteristics.